1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic color copying machine or a color printer, which forms an image by sequentially multi-transferring developing agent images developed on image carriers while conveying a transfer medium by using a semiconductive transfer belt.
2. Description of the Related Art
As one conventional full-coller image forming method, there is a method in that absolute value of the triboelectric charging amount of the second color toner in the second developing unit is controlled larger than that of the first color toner in the first developing unit, such method is shown in such as Jpn. Pat. Appln. KOKAI Publication No. 1-32981.
In this method, a transferring efficiency may be improved, however, this method has a problem that toners are reverse charged easily so as to cause reverse transfer to a photoreceptor.
A multi-transfer system has also the other problem that when a resin sheet, e.g., an OHP (OverHead Projector) sheet, is used as a transfer medium particularly in a high-temperature, high-humidity environment, toner once transferred to the OHP sheet is reverse-transferred to a photoreceptor body in the succeeding transfer unit. The cause of this is considered that an OHP sheet is charged in the preceding transfer unit, and, when this OHP sheet is separated from the photoreceptor body, peel discharge occurs to reversely charge the transferred toner.